The invention relates to buried plastic conduit having internal reinforcement which changes the critical mode of failure from deflection to buckling, and increases load capacity over conventional plastic pipe. The amount of material per lineal foot used in a comparable conventional pipe size is the same but the cross-section is redistributed to achieve increased loading capability.
The primary areas of application include highway and roadway sub-drainage, water supply and monitoring well screens, heat sinks, leachate collection and methane gas collection at landfill sites, and as a conduit for the underground transport of gases and liquids free of solids.
Conduits are typically considered to include small tubes with inside diameters of a fraction of an inch up to conduits tens of feet in diameter, such as used for penstocks in hydro-electric plants. Most pipes are circular or oval in cross-section, however, a variety of cross-sections have been produced. Most pipe cross-sections are circular for the following reasons:
1. Fluids and gases under pressure exert equal (isotropic) loads in all directions; PA1 2. A circular cross-section minimizes friction losses; PA1 3. The volume of the material required in the manufacture of the pipe compared to cross-sectional open area is most efficient; PA1 4. Relative ease of manufacture of thin gauge steel, concrete, and clay, pipe into a circular or oval cross-section; PA1 5. Elimination of stress concentrations; and, PA1 6. Flexibility in installation procedures. PA1 1. Water supply and monitoring well screen; PA1 2. Leachate collection pipes; PA1 3. Methane gas collection pipes; PA1 4. French drains; and, PA1 5. Foundation and pavement underdrains. PA1 1. Septic field disposal conduit; PA1 2. Injection well screen; PA1 3. Dry well screen; and, PA1 4. Air injection system pipes. PA1 1. Graded sand and gravel mixtures placed in holes and trenches surrounding the pipe; PA1 2. Geotextiles wrapped around the pipe; and, PA1 3. The pipe openings themselves may be sized to act as a filter, for example a water well screen. PA1 De=a factor generally taken at a conservative value of 1.5, compensating for the lag or time dependent behavior of the soil/pipe systems (dimensionless) PA1 W=vertical load acting on the pipe per unit of pipe length (lb/in) PA1 r=mean radius of the pipe (in) PA1 E=modulus of elasticity of the pipe materials (psi) PA1 E'=modulus of passive soil resistance (psi) (normally estimated to be 300 psi for soils having Proctor density of 65%, and 700 psi for soils of Proctor density of at least 90%) PA1 K=bedding constant reflecting the support the pipe receives from the bottom of the trench (dimensionless) (a conservative value generally taken to be 0.10) PA1 I=moment of inertia of pipe wall per unit of length (in .sup.4 /in); for any round pipe, I=t.sup.3 /12, where t is the average thickness (in). PA1 E,I and r are as previously defined PA1 F=the recorded load (lb/lineal inch) required to produce a pipe deflection .DELTA.y PA1 .DELTA.y=pipe deflection (in).
Conduits have been fabricated in various shapes including circular, elliptical, rectangular, horseshoe, and other widely used configurations, as required to meet a specific application and as available manufacturing processes permit. Depending upon the material, the methods of manufacture primarily include: rolling; casting; extrusion; welding; and, molding. The conduit of the invention is limited to conduit cross-sections which can be formed by the extrusion process. Extruded polyvinyl chloride plastic pipe is a common product today. Extrusion of PVC plastic into a pipe cross-section was first accomplished many years ago. In recent days, the popularity of PVC for pipe has rapidly grown and reflects its wide range of applications.
Collection conduits are classified herein as those conduits which include perforations, slots or porous walls to allow fluids to pass through the pipe sidewall. Infiltration conduits are similar to collection conduits in that the fluid flows out of the pipe through perforations in the pipe sidewall. Collection conduits also function as transporting conduits permitting collected fluid to be conveyed to some other point along the pipe. Collection pipes include the following:
Infiltration conduits include the following:
Collection conduits are usually encased in a filter to separate the liquids and gases from solids. The conduit walls may or may not be required to function as the filter. Typically filters for collection pipes include the following:
Collection and infiltration conduits differ from transport or conveyance conduits, as the terms are used in the industry, in that the fluid pressure inside the conduit is negligible in comparison to the external load on the pipe. Collection and infiltration conduits are generally buried in the soil, bedrock, or other granular material, where the stresses on the outside of the conduit may be anisotropic or isotropic depending on the orientation of the conduit. Collection and infiltration conduits are also installed in bodies of water, where stresses are isotropic.
Transport conduits include underground pipelines, culverts, tubes, sewers, and tiles conveying gases, liquids and liquid/solid slurries. These conduits differ from collection and infiltration conduits because the sidewalls are impermeable and also the physical design stress for most pressurized transport conduits is the internal fluid or gas pressure. Nearly all plastic pipe is rated for a fluid pressure mainly because burst pressure has been an important factor in the utilization of plastic pipe for such applications.
Culverts and sewers are generally not designed for burst pressure but are designed for gravity flow. A primary consideration is the ability of the pipe to convey liquids containing solids without deposition and clogging. The ability to clean and maintain these types of conduits nearly always dictates that the circular or oval cross-section be used. Other pressurized conduits must from time to time be cleaned with "pigs" to remove scale and bacteria and necessitate the use of circular cross-section for such maintenance.
Many conduits used for the transport of gases and liquids, free of solids, do not however require a circular or oval cross-section. Collection conduits, which incorporate a properly designed filter, convey only liquids. Collection conduits that are subject to clogging and require periodic maintenance are technically failures due to inadequate filter design. Properly designed collection conduits should be maintenance-free and therefore need not maintain a circular cross-section. Similarly, many transport conduits, if clogged, could not be cleared without damaging the pipe in the process. Certain pipe installations simply make maintenance procedures impossible due to inaccessibility.
The present invention includes the utilization of non-circular conduits for those applications where a circular cross-section is not needed for efficient functioning of the pipe. As noted in the foregoing, there are certain benefits inherent in a circular cross-section. However, although horizontally buried pipe with a circular cross-section is effective in resisting both internal and external isotropic stresses, it is yet subject to failure due to anisotropic loading of the surrounding soil. Particularly, when the stress relaxation, or well-known "creep phenomenon" of plastics is considered, the use of a circular cross-sectional area to resist anisotropic loading is a serious design factor. It is accordingly the primary goal of the invention to provide a modified, or redistributed, cross-sectional area for an extruded plastic pipe capable of sustaining increased loading beyond conventional circular cross-sections for the unique loading conditions imposed by underground burial.
Tubing is generally designed to withstand internal pressures without deformation or bursting. Large conduits supported by brackets, pipe racks and piers, are designed with primary consideration for deflection between supports. Submerged and buried pipelines are designed for external pressures induced by the earth load, or in the case of submerged pipe, the hydraulic load. Shallow buried conduits are designed with live-loading as a primary consideration and typically would involve truck and rail traffic, construction equipment wheel loads, and intermittent surcharges due to stockpiled materials. Although internal pressure may be given consideration in the design of buried and submerged pipes, the external loading condition is the critical design criterion.
To achieve reliability and longevity, resistance to the buildup of scale, abrasion, rust, and attack from corrosive groundwater, as well as attack from fluids to be transported, are also necessary considerations in the selection of conduit material.
Polyvinyl chloride, high density polyethylene, and acrylonitrile butadiene styrene, are widely used plastics which achieve good to excellent resistance to commonly encountered chemicals in underground burial applications. Typical of these chemicals are the following: municipal waste by-products including heavy metals and materials with high BOD content; strong and weak mineral acids; oxidizing acids, alkalies; alchohols; ketones; esters; and, vegetable, animal and mineral oils.
The clear-cut approach for the design of a pipeline includes selection of the least expensive pipe material which provides the required resistance to attack from specific chemicals to be conveyed in addition to the resistance to the corrosive environment in which the conduit is to be buried. For example, where drinking water is to be conveyed, the resistance to natural salts and the resistance to rust would be considerations in pipe selection. The acidity of the soil environment must also be taken into account.
Currently available flexible plastic pipe, unless placed in a well compacted granular backfill, is subject to excessive deflections and creep. The dependency on proper bedding for long term successful performance has been a major drawback to the wider application of buried plastic pipe. Less apparent aspects of plastic pipe burial have limited its usage. These include: the capacity of the pipeline to adequately function despite human error in installation; loss of soil support, especially laterally in conventional designs; and, the general capability or vulnerability, of the pipeline to unforeseen chemical, temperature and loading changes in the surrounding environment.
The external pressure on submerged pipeline is directly related to the hydraulic head above the pipe and may conveniently be determined knowing the height above the pipe and the density of the fluid. The fluid pressure acting on the exterior surface, or sidewall, of the pipe acts essentially equally in all directions. Failure of a conduit subjected to exterior hydraulic forces is by collapse of the pipe wall through buckling. On occasion, rapid de-watering of a conduit, with the resulting negative fluid pressure (vacuum), has caused collapse. A circular cross-section is effective in resisting external hydrostatic forces, but only up to a point.
Buried pressurized conduits are less affected by external soil loads than are gravity flow conduits. Gravity flow conduits include sewers, culverts and nearly all collection and infiltration pipes. Whereas most buried pressurized pipeline may be subjected to the full external soil load at the time they are empty, gravity flow conduits are subjected to the full external soil load continuously.
Buried pipes oriented in the vertical direction, for example well screens and well casings, experience stress in a similar fashion to submerged pipes, i.e., nearly isotropic external loading. At commencement of pumping, the load on the well casing increases as the well casing is de-watered. Unequal packing of materials surrounding well screen can result in stress concentrations which may lead to collapse if improperly designed. Plastic well screens are particularly vulnerable to failure by excessive deformation because they are dependent upon the annular backfill space therearound for support.
Horizontally buried pipelines are subjected to anisotropic earth pressures. Theories advanced in the science of soil mechanics are supported by actual measurements and indicate that the vertical pressure from earth loading on a horizontal plane is approximately twice the horizontal pressure on a vertical plane at the same depth below the ground surface. In fluid media pressures are isotropic and act equally in the horizontal and vertical directions at a particular depth. The horizontal pressure at a particular depth is always lower than the vertical load in a soil medium. In the case of free-draining granular materials, for example sand and sand-gravel mixtures commonly used for pipe bedding and backfill, the horizontal earth pressure used for design analysis is approximately 0.3 to 0.5 times the vertical load pressure. In the case of highly compacted clays the horizontal earth pressure approaches 0.7 times the vertical earth pressure. However, pressure required to produce lateral deformation is approximately one to five times the vertical soil pressure at any level. The passive earth pressure (resistance to lateral deformation) of granular backfill, may reach a value of almost five times the vertical soil pressure, and is directly responsible for the well-recognized ability of flexible conduits to resist deformation when embedded in granular backfill.
Flexible conduits, including thin gauge corrugated metal pipe, corrugated and non-corrugated thermoplastics, including PVC and high density polyethylene with an elliptical or circular cross-section, are significantly different from rigid buried conduits. The rigid buried conduit carries, or structurally supports, the overlying soil load. The flexible conduit transfers the vertical load on the conduit through the pipe section to the pipe bedding. The mode of failure for rigid pipes is collapse by excessive tension developed in the sidewall of the pipe. The mode of flexible pipe failure is excessive deflection. The passive resistance of the pipe bedding and backfill is the most critical factor in the resistance of a flexible pipe to deflection and failure. A minimum pipe strength is required to transfer the vertical load to the bedding and is termed the pipe ring stiffness. Beyond the minimum ring stiffness, the deflection of a flexible buried conduit is entirely dependent upon the passive resistance afforded by the pipe bedding and backfill.
The pipe bedding and backfill mass must be statically stable. Static stability of bedding and backfill is very similar to that encountered with a spread footing supporting an above-ground structure. At some point the pipe bedding and backfill "structure" must be supported by the surrounding natural soil or fill. In the case where a pipe bedding and backfill are located in a trench, the passive resistance of the trench sidewalls is the effective foundation for the bedding and backfill. In theory, at a certain loading, namely the passive soil pressure, the trench walls would fail to resist the load imposed by the backfill and bedding "structure".
Flexible buried conduits are commonly positioned in either a projecting mode or trench mode for use in landfills. A so called positive projecting mode of pipe burial disposes the pipe in a mounded backfill above the base of the landfill. The trench mode places the pipe below the base within a trench where the sidewalls of the trench support the bedding and backfill placed around the pipe. In the case of a positive projecting burial, the surrounding backfill must extend far enough beyond the zone of the pipe influence so that the backfill itself does not become unstable. In a projecting mode a bearing capacity-type failure of the backfill is a potential manner of failure.
Rigid conduits include clay tile, cast-iron pipe, concrete pipe, and other conduits which are brittle in nature, fail by collapse at low strain. Rigid pipes of a circular cross-section, are subjected to greater vertical earth pressures than flexible pipes buried at the same level. The rigid pipe is not only subjected to the above loading, but because of its rigidity in the elastic soil medium, an additional vertical soil load is transferred to the pipe. The soil load transferred to a rigid conduit may be several times that imposed on a flexible conduit with a compressibility equal to the adjacent soil. The use of a highly compacted well-graded backfill, with low settlement potential, has been used to minimize the potential for disproportionate settlement between the backfill and the rigid pipe. Under ideal conditions the pipe bedding and backfill should have equal settlement characteristics.
The performance of rigid buried conduits, beyond a certain depth of burial and vertical external loading, depends entirely upon the pipe bedding characteristics. Essentially, once the peak crushing strength, i.e., the resistance to the imposed loading, is exceeded, higher and higher quality gradation and compaction of backfill must be used to effectively limit the load carried by the conduit. Clearly, the more stringent the construction requirements are for backfill the higher the cost of pipe installation becomes and a greater potential for failure exists. Alternatively, increasing the pipe wall thickness will combat increased vertical loads. The use of steel reinforcing and thick walls are obvious measures to provide increased strength for pre-cast concrete conduits.
Flexible conduits are dependent on backfill characteristics but in a different respect than with rigid pipe. Effective design of flexible buried conduits considers the minimum ring stiffness which will act to transmit the vertical load on the pipe to the passive resistance afforded by the pipe bedding and backfill. Recent developments in flexible pipe design have included various techniques to improve the ring stiffness of the pipe material. Among these efforts are corrugated high density polyethylene pipe (HDPE), mortar composition pipe, spiral wall stiffeners, truss wall pipe and corrugated PVC. Each of these pipe wall configurations is designed to improve the ring stiffness of the section by raising the moment of inertia of the pipe section.
In many applications, buried perforated collection and infiltration conduits, constructed of PVC and HDPE, have been used instead of typical porous-wall concrete pipe, open-spaced clay tile, and perforated corrugated metal pipe designs. Perforations consisting of saw-cut slots or drill holes are particularly suitable for plastic pipe. The ease and relative cost of forming the perforations is particularly advantageous in comparison with other pipe materials. For example, this ability to form very thin slots at regular intervals has a definite economic advantage over stainless steel well screens, which have been replaced with slotted PVC pipe for many shallow burial applications. However, there are limitations imposed by the reduction in ring stiffness due to the incorporation of perforations in the pipe wall. The minimum ring stiffness required to resist deep soil burial limits the open area, i.e., perforations, and limits the flow of water into the pipe. This has been a major drawback to increased usage of plastic well screen.
Since reduction of ring stiffness results from perforating pipe walls and tends to reduce the load carrying capability of buried flexible conduits, recent approaches to the problem have attempted to account for this load carrying reduction and have recommended that the design load arbitrarily be increased proportionately to the percentage of reduction in pipe wall area. This approach accounts for the loss in ring stiffness. Buried flexible pipe deflection can be calculated by the widely used Iowa, or Spangler, equation and is presented together with suggested values for its various constants in the 1970 edition of the American Society of Civil Engineers (ASCE) Manual and Practice, No. 37, Chapter 9, Section E, Subsection 1, as follows: ##EQU1## Where: .DELTA.y=horizontal and vertical deflection of the pipe (in)
The term EI in Spangler's equation reflects the pipe's contribution to the total resistance to deflection under load offered by the pipe/soil system. This term, known as the pipe stiffness factor, or ring stiffness, is related to the pipe behavior under parallel plate loading in accordance with ASTM D 2412, "External Loading Properties of Plastic Pipes by Parallel Plate Loading", by the following expression: EQU EI=0.149r.sup.2 (F/.DELTA.y)
Where:
The increase in deflection resulting from lower ring stiffness is apparent from these design equations. It would be desirable to increase the perforated area without the resulting ring stiffness loss imposing such a drastic effect in reducing the loading limit for deflection failure of plastic pipe. In that the design of conventional flexible pipe is controlled by deflection failure, as shown by the above equations, the invention primarily involves changing the mode of critical failure from deflection to buckling, which minimizes reliance on ring stiffness.
With the change from the outmoded open-jointed clay tile and porous wall rigid concrete pipe to modern-day perforated and slotted flexible plastic pipe, the role of the bedding/backfill in the installation of these pipes has changed in yet another manner. The corrosion resistance inherent in plastic pipe may well be negated in some cases by the dependence of pipe performance on the granular backfill where the backfill itself may be subject to attack by acid groundwater. This problem has been found in the design of leachate collection systems for solid waste and hazardous waste disposal systems in areas where carbonate rock is the only type of aggregate readily available. This rock is exceedingly susceptible to acidic groundwater attack. It may dissolve and leave the buried flexible conduit without lateral support. Accordingly, it is a significant goal of the invention to provide an internally reinforced plastic conduit which is not reliant upon the structural properties of the surrounding backfill to prevent failure.
Moreover, it is a concomitant goal of the invention to provide an internally reinforced plastic pipe in which the critical mode of failure is buckling rather than deflection and whereby greater earth loading than a comparable conventional pipe may be imposed without pipe failure.
Also, it is an allied objective of the invention to provide an internally reinforced conduit that incorporates the superior corrosion resistant characteristics of plastics with a unique approach involving a semi-rigid pipe design. It is accordingly a goal of the invention to provide internally reinforced pipe having load/deflection characteristics similar to the surroinding soil by the provision of a reinforcing member designed to more closely match the stress/strain characteristics of the soil than rigid or flexible pipe in the prior art.
It is an important goal of the invention to rely on the buckling resistance of an internally reinforcing stem member to resist deflection. The objective involves providing that the mode of critical failure is buckling of internal reinforcement and not deflection of pipe sidewalls.
It is another object of the invention to provide an extruded plastic conduit having internal reinforcement which allows the imposition of a larger earth loading than a comparably sized conventional pipe and wherein the amount of plastic used per lineal foot is substantially identical to, or less than, the amount used in the conventional pipe. This goal includes re-arranging the pipe cross-section.
It is also an important goal of the invention to provide an internally reinforced plastic pipe which permits a plurality of reinforcing stems for use in isotropic loading conditions, such as a well screen or monitoring well, and eliminate the need for the more costly utilization of stainless steel. This is an important consideration where a high percentage of open area is required to permit inflow of water.
It is an allied objective of the invention to provide an internally reinforced extruded plastic pipe which is capable of sustaining greater loading, even when perforated, than conventional non-reinforced pipe that is non-perforated.